ARTICLE pubs.acs.org/IECR
Preparation of Poly(p-dioxanone)/Sepiolite Nanocomposites with Excellent Strength/Toughness Balance via Surface-Initiated Polymerization Zhi-Cheng Qiu, Jing-Jing Zhang, Ying Niu, Cai-Li Huang, Ke-Ke Yang,* and Yu-Zhong Wang Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MoE), National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China ABSTRACT: Novel nanocomposites based on aliphatic poly(ester-ether) [poly(p-dioxanone), PPDO] and fibrous clay (sepiolite) were successfully prepared by surface-initiated polymerization of p-dioxanone (PDO) in the presence of organo-modified sepiolite (OSEP). The existence of OSEP-grafted PPDO in nanocomposites, generated via surface-initiated polymerization from the hydroxyl group-abundant OSEP surface, greatly contributes to improving the performance of these nanocomposites. The dispersion uniformity of OSEP nanofibers in PPDO matrix was confirmed by scanning electron microscope (SEM) observations. Crystallization rate increased with increasing OSEP loading. Surprisingly, only OSEP nanofibers bundles acted as heterogeneous nucleating agents during the latter stage of PPDO crystallization. The incorporation of OSEP significantly enhanced the modulus and viscosity of nanocomposites, whereas nanocomposites still presented liquid-like behavior as neat PPDO. The exciting aspect of this research is that nanocomposites were reinforced and toughened by the addition of OSEP nanofibers.
1. INTRODUCTION Aliphatic poly(ester-ether), well-known for its excellent biodegradability and mechanical performance, has received growing attention from industry and academia during the past decade due to its potential applications especially in environmental fields.13 The presence of an ether bond in its molecular structure confers greater flexibility on aliphatic poly(ester-ether) in comparison to aliphatic polyester such as poly(lactic acid) (PLA) and poly(βhydroxybutyrate) PHB. Poly(p-dioxanone) [poly(1,4-dioxan-2one), PPDO] is a typical aliphatic poly(ester-ether) with excellent biodegradability, bioabsorbability, biocompatibility, and flexibility.4 As other aliphatic poly(ester-ether), PPDO has drawbacks of low crystallization rate and low melt strength, which limits its applications and processing. Several approaches, such as copolymerization with other monomers47 and blending with other polymers,811 have been developed to improve the properties of PPDO. The addition of inorganic nanoparticles is also an attractive way.1222 Sepiolite is a typical fibrous clay with Si12O30Mg8(OH)4(H2O)4 3 8H2O as the unit cell formula.23,24 The structure of sepiolite, formed by two tetrahedral silica sheets enclosing a central sheet of octahedral magnesia, relates to 2:1 layered structure of smectite clays except that the layers lack continuous octahedral sheets. Therefore, rectangular channels occur in the longitudinal direction of strips. Compared with montmorillonite (MMT), sepiolite has very high specific surface area (BET 374 ( 7 m2/g), fibrous morphology, and high-density SiOH.25 Hydroxyl groups on the surface of nanoparticles can be utilized as polymerization initiators for the ring-opening polymerization of lactone, leading to polylactone chains grafting onto the nanoparticle surface.1822,2631 Yoon et al.26 reported poly(1,5dioxepan-2-one)/SiO2 hybrid nanoparticles in which poly(1,5dioxepan-2-one) molecular chains were grown directly from SiOH groups of SiO 2 nanoparticles by surface-initiated r 2011 American Chemical Society
ring-opening polymerization of 1,5-dioxepan-2-one. Carrot et al.27 prepared poly(ε-caprolactone) (PCL)/CdS hybrid nanoparticles by ring-opening polymerization, which was initiated from hydroxyl groups spread over the surface of CdS nanoparticles. This work sets out to investigate the incorporation of fibrous clay on properties of an aliphatic poly(ester-ether) matrix. Aliphatic poly(ester-ether)/fibrous clay (PPDO/sepiolite) nanocomposites with highly improved performance were successfully prepared by ring-opening polymerization and surface-initiated polymerization of p-dioxanone (PDO), utilizing the abundant SiOH groups of organically modified sepiolite (OSEP) surface. Systematic investigation on the structure, morphology, and properties of the nanocomposites was also performed.
2. EXPERIMENTAL SECTION 2.1. Materials. PDO, provided by the Pilot Plant of the Center for Degradable and Flame-Retardant Polymeric Materials (Chengdu, China), was dried over calcium hydride (CaH2) at room temperature under reduced pressure until the water content was less than 300 ppm and distilled under reduced pressure before use. Triethylaluminum (AlEt3) was purchased from J&K Chemical Ltd. Sepiolite with a cation exchange capacity (CEC) of 30 meq/100 g, modified by alkyl benzyl dimethyl ammonium chloride (PPDO is a kind of polar ester resin, so absorption of alkyl benzyl dimethyl ammonium on the surface of sepiolite will enhance compatibility between sepiolite and PPDO matrix), was supplied by Tolsa SA (Spain), and the Received: January 26, 2011 Accepted: July 13, 2011 Revised: May 6, 2011 Published: July 13, 2011 10006
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Table 1. Polymerization of PDO in the Presence of OSEPa sample
OSEP content (wt %)
[η] (dL/g)
conv (%)
PPDOb
0
1.94
90.8
PPDO/1%OSEP
1
2.61
90.7
PPDO/2%OSEP
2
3.07
90.6
PPDO/3%OSEP
3
3.18
90.7
PPDO/4%OSEP
4
3.00
89.9
PPDO/5%OSEP
5
2.82
89.2
a
General reaction conditions: [monomer]0/[AlEt3] = 400, time = 4 h, temperature = 50 °C. b Neat PPDO was synthesized under the same fabrication condition as nanocomposites.
organic content (determined by thermogravimetric analysis) in the OSEP was about 6.14 wt %. 2.2. Preparation of the PPDO/OSEP Nanocomposites. PPDO/OSEP nanocomposites were prepared by the in situ ring-opening polymerization of PDO catalyzed by AlEt3 in the presence of OSEP. The desired amount of OSEP was placed in a flame-dried and nitrogen-purged round-bottom flask equipped with a stirrer and a rubber septum. Then the flask was dried by flame and purged by nitrogen three times. The quantitative PDO was added with an injector, and the reaction medium was stirred at 50 °C for 3 h and another 1 h under ultrasonic action. Then, the catalyst, a solution of AlEt3 in heptane, was added. The molar ratio of the initial monomer to AlEt3 was 400. The polymerization was conducted at 50 °C for 4 h. 2.3. Characterization. As the conventional solvents such as chloroform and tetrahydrofuran used in gel permeation chromatography (GPC) measurements cannot dissolve PPDO with high molecular weights, the intrinsic viscosity [η] of neat PPDO and its nanocomposites was measured in phenol/1,1,2,2-tetrachloroethane (1:1 v/v) solution using an Ubbelohde viscometer maintained at 30 °C. The test results are presented in Table 1. Fourier transform infrared spectrum (FT-IR) analysis was performed with a Nicolet 670 FTIR spectrometer (Thermo Nicolet, USA) for a scanning coverage from 4000 to 400 cm1 at a spectral resolution of 4 cm1 using 32 scans. OSEP, neat PPDO, and sepiolite isolated from PPDO/OSEP nanocomposites were mixed with dried potassium bromide (KBr) powder and compressed into a disk, respectively. Sepiolite isolated from PPDO/OSEP nanocomposites was obtained via thorough washing nanocomposites with dimethyl sulfoxide to remove all PPDO homopolymers at 80 °C. X-ray diffraction (XRD) analyses were performed on an X’pert diffractometer (Philips, Netherlands) which had an X-ray generator of 3 kW, graphite monochromatic, Cu KR radiation (wavelength = 1.5406 Å) and was operated at 40 kV and 20 mA. XRD pattern of OSEP was recorded directly from powders, and those of neat PPDO and its nanocomposites were evaluated from films. The samples were scanned at room temperature from 2° to 30° at a scanning rate of 2°/min. The morphologies of the fracture surfaces of various PPDO/ OSEP nanocomposites, which were broken off in liquid nitrogen and coated with gold, were observed by a FEI Inspect F Scanning Electron Microscope (Philips, Netherlands) using 5 kV accelerating voltage. The crystallization behavior of PPDO and its nanocomposites was studied by DSC Q200 (TA Instruments, USA). For nonisothermal crystallization, the samples first were kept isothermally at 140 °C for 5 min to erase previous thermal history, and
quenched to 50 °C, then heated up to 140 at 10 °C/min, held at 140 °C for 3 min to erase any thermal history, and cooled to 10 °C at different constant cooling rates ranging from 1.25 to 10 °C/min. The crystallization peak temperature was obtained from the cooling traces. For isothermal crystallization, the samples were heated to 140 °C for 5 min to erase previous thermal history, and then quenched to the isothermal crystallization temperature (Tc), held until the isothermal crystallization completed. The exothermal traces were recorded for the later data analysis. Polarized optical microscopy (POM) studies were carried out with an ECLIPSE LV100POL microscope (Nikon, Japan) in conjunction with an HSC621 V hot stage (Instec, USA). The specimens were heated to 140 °C on a hot stage and held at that temperature for 3 min, and then quickly cooled to crystallization temperature. The photographs were taken by a digital camera. Thermogravimetric analysis (TGA) was performed on a TG 209 F1(NETZSCH, Germany) at a heating rate of 10 °C/min under a steady flow of nitrogen (50 mL/min), and the temperature range was from room temperature to 450 °C. The rheological measurements were undertaken using a Bohlin Gemini 200 instrument (Malvern, UK) with 25-mmdiameter parallel-plate geometry at 130 °C. The test samples were pressed into 1-mm-thick plates at 140 °C. Dynamic oscillatory shear measurements were performed when the angular frequency (ω) range and strain used during testing were 0.01100 rad 3 s1 and 1%, respectively. Steady shear viscosity measurements were performed at shear rates 0.00110 s1. The dynamic mechanical analysis (DMA) was conducted on a DMA Q800 (TA Instruments, USA) in the tension mode using rectangular samples. The samples were subjected to a cyclic tensile strain with an amplitude of 0.2% at a frequency of 1 Hz and a static force of 0.1 N. The temperature range was from 50 to 90 °C with a heating rate of 3 °C/min. The dried neat PPDO and its nanocomposites pellets were injection-molded using HAAKE MinJet (Thermo electron corporation, USA) operated at 160 °C with a mold temperature of 30 °C. The mechanical performance was measured by SANS CMT4104 universal testing machine (SANS Group, China) at a crosshead speed of 25 mm/min with GB/T 1040.22006 method at room temperature. At least five specimens were tested in each group and the reported values of each group reflected an average of five specimens.
3. RESULTS AND DISCUSSION 3.1. Preparation of the PPDO/OSEP Nanocomposites. In this work, AlEt3 was used as catalyst for the ring-opening polymerization of PDO. Due to OSEP with abundant SiOH on its surface, the surface-initiated reaction will also occur and OSEP-grafted PPDO is produced.1822 Figure 1 shows the schematic representation of the procedure for formation of PPDO/OSEP nanocomposites. Prior to the polymerization, AlEt3 not only reacted with trace amounts of water in the PDO monomer but also with SiOH on OSEP surface to form aluminum-alkoxide-active species, respectively. Then PDO polymerization was carried out by inserting the PDO monomer into the aluminum-alkoxide-active species. Consequently, PPDO synthesized by this method was composed of nongrafted PPDO and OSEP-grafted PPDO. This speculation was confirmed by FTIR spectroscopy. Curve b in Figure 2 shows FTIR spectra of sepiolite isolated from PPDO/OSEP nanocomposites. Peaks 10007
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Figure 1. Schematic representation of the procedure for formation of PPDO/OSEP nanocomposites.
Figure 2. FTIR spectra of (a) OSEP, (b) sepiolite isolated from PPDO/ OSEP nanocomposites, and (c) neat PPDO.
characteristic of PPDO are observed in the isolated sepiolite: 1749 and 1735 cm1 (CdO stretching), 1131 and 1207 cm1 (COC stretching), and 2927 cm1 (CH stretching).32 It is verified that PPDO chains have been grafted from the surface of sepiolite. In addition, OSEP content and the intrinsic viscosity [η] of all samples produced in the present work are listed in Table 1. Obviously, [η] of PPDO/OSEP nanocomposites is higher than that of neat PPDO under the same fabrication
condition. This also could be regarded as an indirect proof of the existence of OSEP-grafted PPDO in nanocomposite. 3.2. Morphology and Structure of PPDO/OSEP Nanocomposites. Figure 3 shows the SEM photographs of the fractured section of various PPDO/OSEP nanocomposites. It can be seen that nanosized sepiolite fibers distributed uniformly in the fractured section of PPDO/OSEP nanocomposites samples. Only a very small amount of tiny OSEP nanofiber bundles can be observed when the content of OSEP is up to 3 wt %. In addition, the interface between OSEP nanofibers and PPDO matrix is vague. Most of the OSEP nanofibers were fractured and only a few nanofibers were pulled out from the PPDO matrix. These phenomena indicate a strong interfacial adhesion between OSEP nanofibers and PPDO matrix. However, both OSEP and nanocomposites have diffraction peak at 2θ = 7.3°, corresponding to d-spacing of 1.23 nm (Figure 4). Unlike smectite clays, the layers of sepiolite are linked by covalent bonds and sepiolite is fibrous in nature. The characteristic diffraction peak (110) is due to the intrinsic axis structure of fibrous crystals. Therefore, even the completely exfoliated fibrous crystals of sepiolite hardly have an effect on the location of diffraction peak (110) in the XRD pattern. 3.3. Crystallization Behaviors of PPDO/OSEP Nanocomposites. 3.3.1. Spherulitic Morphology and Structure. The spherulitic morphology of PPDO and its nanocomposites was observed by POM. Figure 5 presents spherulitic morphology of samples crystallized at 70 °C with different crystallization time. The birefringent ring-banded spherulitic structure can be observed for neat PPDO and all PPDO/OSEP nanocomposites samples. One can also be seen from Figure 5 that crystallization 10008
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Figure 3. SEM images: (a) PPDO/1%OSEP ( 50 000), (b) PPDO/3%OSEP ( 50 000), (c) PPDO/5%OSEP ( 50 000). The bright entities are the cross section of OSEP nanofiers, and the dark areas are the matrices.
Figure 4. XRD patterns of OSEP powders, neat PPDO and its nanocomposites.
process of PPDO/1%OSEP nanocomposites is similar to neat PPDO. This phenomenon indicates that uniformly dispersed OSEP nanofibers cannot act as nucleating agent effectively during the crystallization process of PPDO. When OSEP content reached or went beyond 3 wt %, however, heterogeneous nucleation phenomenon appears in the latter stage of PPDO crystallization, illustrating that the crystallization process of nanocomposites is divided into two stages. In the early stage of crystallization nanocomposites are mainly homogeneous nucleations of PPDO molecular chains, while heterogeneous nucleation of OSEP nanofiber bundles becomes predominant in the latter stage of crystallization. This interesting phenomenon may be due to the unique structure of sepiolite nanofiber, which possesses high-density SiOH on the surface. The hydrogen bonds between SiOH can be broken and replaced by hydrogen bonds between SiOH and PPDO molecular chains. Furthermore, it was approved that the surface of OSEP possessed grafted PPDO polymer chains, so the motion of the PPDO molecular chains around OSEP nanofibers was restricted seriously, and it had to overcome an energy barrier to drive the nucleating and growing. After the early stages of crystallization, on the other hand, the free volume of PPDO molecules increases because a part of the free PPDO molecular chains have densely folded into lamellae, and the mobility of remaining PPDO molecular chain increases simultaneously. Moreover, the heterogeneous nucleating agents for PPDO crystallization need to have sufficient size.14 Therefore, after an induction period, PPDO molecular chains
have enough mobility to fold on the surface of OSEP nanofiber bundles to form nucleus and arrange into lamellae. Figure 4 also gives the information of crystal structure in PPDO matrix; it clearly shows that various PPDO/OSEP nanocomposites have the same characteristic diffraction peaks at 2θ = 21.9° 23.7°, and 29.3° as neat PPDO does,33 which are attributed to the reflections of (210), (020), and (310), respectively. The result suggests that there is no significant effect of the OSEP on the crystal structure of PPDO matrix in nanocomposites. 3.3.2. Nonisothermal Crystallization Behavior. Figure 6 shows the DSC heating traces of neat PPDO and its nanocomposites with different OSEP contents. All the relevant enthalpies and transition temperatures obtained from Figure 6 are listed in Table 2. And the absolute degrees of crystallinity (χc) were calculated by the ratio of the melting enthalpy (ΔHm) of samples to that of 100% crystalline PPDO (141.2 J 3 g1).34 According to the data listed in Table 2, the cold crystallization temperature (Tc) of samples gradually decreases with increasing content of OSEP while Tm slightly increases. Moreover, χc also increases modestly with increasing OSEP content. All these clearly indicate that OSEP in nanocomposites acts as heterogeneous nucleating agents and the addition of OSEP restricts the mobility of PPDO molecular chains, due to the existence of PPDO-grafted OSEP. To investigate the effect of cooling rate on the nonisothermal melt crystallization, Figure 7 summarizes the variation of crystallization peak temperature (Tp) with cooling rate (Φ) for neat PPDO and its nanocomposite. Generally, Tp always shifts to high temperature range with the addition of clay. In this case, although, Tp shifts to high temperature with increasing OSEP contents in the nanocomposites, it can not be ignored that Tp of nanocomposites is obviously lower than that of neat PPDO at high cooling rate. This interesting phenomenon may be ascribed to the existence of PPDO-grafted OSEP. At high cooling rate, PPDO molecular chains are difficult to move and arrange, moreover, the existence of PPDO-grafted OSEP hinders the movement of PPDO molecular chains, resulting PPDO molecular chains are difficult to form nucleus. Consequently, the nucleation effect of OSEP is limited. 3.3.3. Isothermal Crystallization Behavior. The overall isothermal crystallization of neat PPDO and its nanocomposites with various OSEP contents were investigated by DSC in a temperature range from 55 to 75 °C. Figure 8a shows the plots of relative crystallinity versus crystallization time for neat PPDO and its nanocomposites at 70 °C. The Avrami equation is used to analyze the isothermal crystallization kinetics of neat PPDO and its nanocomposites. It assumes that the relative degree of crystallinity Xt develops as a function of crystallization time t as follows:35 1 Xt ¼ expð kt n Þ 10009
ð1Þ
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Figure 5. Crystallization process of neat PPDO and its nanocomposites at crystallization temperature 70 °C.
Figure 6. DSC traces of neat PPDO and its nanocomposites after meltquenching at a heating rate of 10 °C/min.
Table 2. DSC Data for Neat PPDO and Its Nanocomposites sample PPDO
Tg (°C) Tc (°C) ΔHc (J/g) Tm (°C) ΔHm (J/g) χca (%) 11.48
47.47
46.70
103.97
50.82
36.00
PPDO/1%OSEP 10.59
36.67
46.30
104.19
50.98
36.47
PPDO/3%OSEP 10.67
35.23
46.06
104.23
52.32
38.20
PPDO/5%OSEP 10.80
33.97
45.58
104.75
52.86
39.41
Absolute degree of crystallinity, χc (%) = 100 ΔHm/(1 j) ΔHmo. j is the weight fraction of OSEP. a
where Xt is the relative degree of crystallization, n is the Avrami exponent, and k is a composite rate constant involving both
Figure 7. Effect of cooling rates on the crystallization peak temperatures for neat PPDO and its nanocomposites.
nucleation and growth rate parameters. These parameters are related to the crystallization half-time t0.5. The value of t0.5 is calculated by the following equation: 1=n ln 2 ð2Þ t0:5 ¼ k All the isothermal crystallization kinetics parameters of PPDO and its nanocomposites are summarized in Table 3. The average values of n are around 2.53 for neat PPDO, 2.80 for PPDO/1% OSEP, 3.14 for PPDO/3%OSEP, and 3.29 for PPDO/5%OSEP. In the other words, the values of n are close to 3 for the isothermal 10010
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Table 3. Crystallization Kinetic Parameters for Neat PPDO and Its Nanocomposites Tc (°C)
sample PPDO
PPDO/1%OSEP
PPDO/3%OSEP
PPDO/5%OSEP
Figure 8. Effect of OSEP on the crystallization of PPDO at 70 °C: (a) development of relative crystallinity with crystallization time; (b) the Avrami plots.
crystallization of both neat PPDO and its nanocomposites, suggesting spherulitic growth followed by a thermal nucleation. Moreover, the nonintegral n value between 2 and 4 may reflect the crystal branching and/or mixed crystal growth and nucleation mechanism.36 And the average values of n increase with increasing content of OSEP, revealing that the incorporation of OSEP causes heterogeneous athermal nucleation followed by higher crystal branching in spherulitic growth. PPDO is observed to crystallize according to the spherulitic growth followed by heterogeneous athermal nucleation in the latter stage of crystallization in the POM micrograph when the content of OSEP is up to 3 wt % in nanocomposites (Figure 5). It also has to be mentioned that the slop of Avrami plots diverges from the initial part at high Xt (Figure 8b), due to heterogeneous nucleation of OSEP nanofiber bundles in the latter stage of PPDO crystallization. Therefore, it can be concluded that the DSC results are in good agreement with the POM morphology studies. Usually, the crystallization rate can also be easily described by the reciprocal of t0.5. Figure 9 illustrates Tc dependence of 1/t0.5 for neat PPDO and its nanocomposites with different OSEP loading. As shown in Figure 9, the values of 1/t0.5 decrease with increasing Tc for all samples. This result indicates that the overall isothermal crystallization rate decreases with increasing Tc.
n
k (minn)
t0.5 (min) 1/t0.5 (min1)
55
2.51 2.704 103
60 65
2.53 1.683 103 2.53 7.870 104
10.77 14.57
0.09285 0.06863
70
2.46 3.013 104
23.26
0.04299
75
2.60 5.236 105
38.76
55
3.19 7.925 103
4.071
0.2456
60
2.76 8.035 103
5.043
0.1983
65
2.77 3.631 103
6.641
0.1506
70
2.67 6.281 104
13.78
0.07257
75 55
2.62 1.159 104 3.35 3.673 102
27.67 2.402
0.03614 0.4163
60
3.28 1.127 102
3.513
0.2847
65
3.19 3.221 103
5.381
0.1858
70
2.91 1.014 103
9.422
0.1061
75
2.95 1.239 104
55
3.21 1.791 101
1.525
0.6557
60
3.46 2.239 102
2.698
0.3706
65 70
3.41 4.055 103 3.21 9.183 104
4.524 7.885
0.2210 0.1268
75
3.17 1.618 104
9.106
18.56
14.00
0.1098
0.02580
0.05388
0.07143
Furthermore, the values of 1/t0.5 are raised monotonically with increasing OSEP content at a given Tc, especially at low Tc, indicating that the addition of OSEP accelerates the crystallization process of PPDO and OSEP content has a significantly effect on accelerating the crystallization of PPDO, especially at low Tc. To study the effect of the presence of OSEP and their contents on the crystallizability of PPDO in nanocomposites quantitatively, the crystallization rate parameter k was used to determine the crystallization activation energy as the following Arrhenius equation:37 1 ΔEa ðln kÞ ¼ ln k0 n RTc
ð3Þ
Where k0 is a temperature-independent preexponential factor; ΔEa is the crystallization activation energy, which consists of the transport activation energy and the nucleation activation energy; R is the universal gas constant. The plots of 1/n(lnk) versus 1/Tc for neat PPDO and its nanocomposites with various OSEP loadings are shown in Figure 10. The values of ΔEa are calculated as 68.88, 92.56, 97.10, and 104.66 KJ/mol for neat PPDO, 1 wt %, 3 wt %, and 5 wt % PPDO/OSEP nanocomposites, respectively. It is obvious that the values of ΔEa drastically decrease with increasing OSEP contents, which indicates again the addition of OSEP induces the heterogeneous nucleation. The equilibrium melting point T0m of neat PPDO and its nanocomposites can be determined from the plot of Tm versus Tc through HoffmanWeeks equation,38 which is given as Tc 1 þ 1 Tm0 Tm ¼ ð4Þ γ γ where T0m is the equilibrium melting point, γ is a constant relating to crystal size and perfection, and 1/γ is the crystal stability parameter. The equilibrium melting point can be obtained from 10011
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Figure 9. Temperature dependences of 1/t0.5 for neat PPDO and its nanocomposites at various Tc.
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Figure 11. HoffmanWeeks plot for the estimation of the equilibrium melting point temperatures for neat PPDO and its nanocomposites.
Figure 10. Arrhenius plots of 1/n(lnk) versus 1/Tc for neat PPDO and its nanocomposites.
the intersection of this line with the Tm = Tc, which is shown in Figure 11. The value of extrapolated T0m is 121.23 °C for neat PPDO, 119.26 °C for PPDO/3%OSEP, and 118.55 °C for PPDO/5%OSEP. Obviously, the values of T0m decrease with increasing OSEP loading, suggesting the crystalline structure of PPDO/OSEP nanocomposites is less perfect than that of neat PPDO. This phenomenon may be due to the presence of more heterogeneous nucleation induced by OSEP nanofiber bundles and more restriction of PPDO molecular chains packed among OSEP nanofibers, which lead to an inferior crystalline nature that caused T0m to shift to lower temperatures.39,40 3.4. Rheological Behaviors of PPDO/OSEP Nanocomposites. The measurement of rheological properties of polymeric materials in a molten state is crucial to gain fundamental understanding of the processability and the structureproperty relationship for these materials. In this section the melt rheological behaviors of neat PPDO and its nanocomposites are discussed.
Figure 12. Frequency dependence of storage modulus[G0 ], loss modulus [G00 ], and complex viscosity[|η*|] of PPDO/OSEP nanocomposites and PPDO matrix at 130 °C.
3.4.1. Dynamic Oscillatory Shear Measurements. The linear dynamic viscoelastic master curves for neat PPDO and its nanocomposites are shown in Figure 12. The storage modulus (G0 ), loss modulus (G00 ), and complex viscosities (|η*|) exhibit a 10012
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Figure 13. Complex viscosity versus OSEP content at different frequencies.
Table 4. Slopes of G0 (ω) and G00 (ω) at Low Frequencies for Neat PPDO and Its Nanocomposites sample
0
00
G (ω)
G (ω)
PPDO
1.09
0.96
PPDO/1%OSEP
0.99
0.85
PPDO/3%OSEP
1.03
0.83
PPDO/5%OSEP
1.05
0.78
monotonic increase with increasing OSEP loading at all frequencies, especially at low frequencies. Figure 13 shows |η*| versus OSEP content at different frequencies. When ω e 10 rad/s, |η*| of samples do not increase linearly with increasing OSEP content. There is a steep slop between 0 and 1 wt %; and between 1 and 5 wt % the slope is lower. When ω = 100 rad/s, the increase of |η*| is nearly linear with increasing OSEP content. The dramatic growth of |η*| with only 1 wt % OSEP addition is mainly caused by the entanglement occurred between the OSEPgrafted PPDO molecules and PPDO matrix molecules. Terminal zone slopes of G0 and G00 were estimated for all samples at frequencies below 1 rad/s and are reported in Table 4. The terminal zone frequency dependence of G0 for nanocomposites is nearly identical to neat PPDO, depending as ω1.09 for neat PPDO to ω1.05 for PPDO/5%OSEP. And the frequency dependence of G00 for samples decreases slightly with increasing OSEP loading from ω0.96 for neat PPDO to ω0.78 for PPDO/5%OSEP. The results indicate the melt of nanocomposites still exhibits liquid-like behavior at low frequencies as neat PPDO. 3.4.2. Steady Shear Measurements. The steady shear rheological behavior of neat PPDO and its nanocomposites is shown in Figure 14. The steady shear viscosity of the PPDO/OSEP nanocomposites is enhanced considerably at low shear rates and increases monotonically with increasing OSEP content at a fixed shear rate, showing that the effect of OSEP nanofibers on the shear viscosity is very pronounced. This may be explained by the existence of PPDO-grafted OSEP in the system contributing to the great enhancement in shear viscosity of nanocomposites. 3.5. Thermal Stability and Dynamic Mechanical Behavior of PPDO/OSEP Nanocomposites. Figure 15 shows TGA and
Figure 14. Steady shear viscosity of pure PPDO and its nanocomposites as a function of shear rate at 130 °C.
DTG curves of neat PPDO and its nanocomposites. The 5 wt % weight loss temperature (T5%) and the maximum weight loss rate temperature (Tmax) of samples are listed in Table 5. T5% of samples decreases linearly with increasing amount of OSEP with a maximum decrease of 12.4 °C from the value of neat PPDO to 5 wt % OSEP. And from Figure 15b, it can be seen the weight loss rate of samples obviously increases with increasing OSEP content when the decomposition temperature is below the T5% of neat PPDO. It is well-known the decomposition of PPDO proceeds by unzipping depolymerization as main reaction.33 However, in the initial stages, the decomposition of PPDO is mainly led by the random reactions because its repeated aliphatic ester-ether structure is relatively easy to hydrolyze.41 Sepiolite possesses absorbed water and high-density SiOH on its surface, which accelerates the decomposition of ester group in PPDO. Nevertheless, it also can be noticed that Tmax of nanocomposites is higher than that of neat PPDO. This result may be attributed to the uniform dispersion of OSEP nanofibers and the porous structure of sepiolite nanofibers, which can absorb the volatile products during PPDO degradation. Figure 16 shows the dynamic storage modulus E0 and loss factor tanδ as a function of temperature for neat PPDO and its nanocomposites. It can be seen from Figure 16a that the storage modulus of the nanocomposites raises with increasing OSEP loading, indicating that the incorporation of OSEP into PPDO matrix remarkably enhances stiffness and has a good reinforcing effect. Figure 16b reveals the effect of OSEP on loss factor tanδ of nanocomposites. The value of the glass transition temperature Tg (the temperature of tanδ peak) is 2.86 °C for neat PPDO, 1.54 °C for PPDO/1%OSEP, 3.02 °C for PPDO/3%OSEP, and 1.8 °C for PPDO/5%OSEP. Evidently, Tg decreases with the loading of 1 and 3 wt % OSEP into PPDO, and then increases as the loading of OSEP continuously increased to 5 wt %. This decrease in Tg is probably due to the plasticizing effect from the organic modifier and low molecular weight OSEP-grafted PPDO within OSEP.42,43 However, Tg of nanocomposites increases with increasing OSEP content from 35 wt %, perhaps suggesting that the addition of more OSEP causes more steric hindrance and reduces the mobility of polymer chains. 10013
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Figure 15. TGA and DTG curves of neat PPDO and its nanocomposites: (a) TGA curves, (b) DTG curves.
Table 5. TGA Data for Neat PPDO and Its Nanocomposites T5% (°C)
Tmax (°C)
T90% (°C)
PPDO
197.1
239.1
243.4
PPDO/1%OSEP
194.1
240.9
246.5
PPDO/3%OSEP
191.2
246.0
251.0
PPDO/5%OSEP
184.7
246.6
249.1
sample
3.6. Tensile Properties of PPDO/OSEP Nanocomposites. The tensile properties of the nanocomposites are mainly affected by three factors as follows: the aspect ratio of the filler, the degree of dispersion of the filler in matrix, and the interfacial interaction between filler and matrix. Table 6 summarizes the tensile properties of PPDO/OSEP nanocomposites with various OSEP contents. Apparently, the yield strength, tensile strength, and Young’s modulus of the samples increase significantly by the addition of OSEP. OSEP content of only 3 wt % leads to an increase in yield strength from 28.3 MPa for neat PPDO up to 35.7 MPa and an increase in Young’s modulus from 370 MPa for neat PPDO up to 521 MPa. Generally, the stiffness improvement is always accompanied by a sacrifice in the toughness. Unexpectedly, the elongation at break of nanocomposites was improved obviously, increasing from 280% for neat PPDO to 441% for PPDO/3%OSEP nanocomposites. The increase in stiffness can not only be explained by uniformly dispersed OSEP nanofibers with high aspect ratio and high intrinsic stiffness, but also can be ascribed to the strong interface interaction between OSEP nanofibers and PPDO matrix. Due to the existence of OSEP-grafted PPDO chains, the surfaces of OSEP nanofibers have chemical nature similar to PPDO matrix, thereby there is strong interfacial adhesion between OSEP nanofibers and PPDO matrix. Moreover other strong interaction such as chains entanglement and cocrystallization of OSEP-grafted PPDO chains with PPDO matrix could strengthen the interphase between OSEP nanofibers and PPDO matrix. Therefore, stress can be effectively transferred from the PPDO matrix to the OSEP nanofibers, resulting in the improved stiffness of nanocomposites. Similar to the reinforcement
Figure 16. DMA curves of neat PPDO and its nanocomposites: (a) storage modulus as a function of temperature; (b) tanδ as a function of temperature. 10014
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Table 6. Mechanical Properties of Neat PPDO and Its Nanocomposites sample
yield strength MPa
strain at yield %
tensile strength MPa
elongation at break %
Young’s modulus MPa 370 ( 29
PPDO
28.3 ( 0.6
39.1 ( 1.1
46.1 ( 2.5
280 ( 25
PPDO/1%OSEP
30.3 ( 0.9
39.2 ( 0.6
56.1 ( 1.3
391 ( 19
339 ( 6
PPDO/3%OSEP
35.7 ( 0.9
43.1 ( 1.6
62.7 ( 1.4
441 ( 14
521 ( 13
PPDO/5%OSEP
35.0 ( 1.2
40.6 ( 1.3
54.7 ( 1.6
273 ( 13
634 ( 19
mechanism, interfacial interaction also dominates toughness of nanocomposites. Strong interface interaction between OSEP nanofibers and PPDO matrix facilitates enhancement of the resistance to crack propagation, so that the nanocomposite can tolerate higher strain as loading. The same effect can also be observed from other studies on polymer-grafted nanoparticles incorporation into miscible polymer matrix.4247 However, at the higher OSEP content of 5 wt %, a decrease in both the yield strength and the elongation at break can be observed. The agglomeration of nanofibers at higher OSEP content results in the reduction of the effective surface area which can undergo matrixfiller interactions.4850
4. CONCLUSION We have successfully prepared PPDO/OSEP nanocomposites with excellent strength/toughness balance by in situ ring-opening polymerization. The existence of OSEP-grafted PPDO in nanocomposites, generated via surface-initiated polymerization from the hydroxyl-group-abundant OSEP surface, was confirmed by FTIR. SEM observations demonstrated OSEP nanofibers were well dispersed in PPDO matrix and there was strong interfacial interaction between OSEP nanofibers and PPDO matrix. Crystallization rate increased with increasing OSEP loading. Due to the motion of the PPDO molecular chains around OSEP nanofibers being seriously restricted, only OSEP nanofibers bundles with sufficient size could act as heterogeneous nucleating agents during the latter stage of PPDO crystallization. T5% of nanocomposites decreased with increasing of OSEP content, while compared with neat PPDO, Tmax of nanocomposites was enhanced. These results were attributed to the uniform dispersion of OSEP nanofibers and the porous structure of sepiolite nanofibers. The incorporation of OSEP significantly enhanced the modulus and viscosity of nanocomposites, whereas nanocomposites still presented liquid-like behavior as neat PPDO. The exciting aspect of this research is that nanocomposites were reinforced and toughened by the addition of OSEP nanofibers, which proved fibrous clay was an ideal filler for aliphatic poly(ester-ether). ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: +86-28-85410755. Fax: +86-28-85410284.
’ ACKNOWLEDGMENT We gratefully acknowledge the financial support of National Science Foundation of China (50873064), the program for New Century Excellent Talents in University (NCET-07-0586), and the Funds for Young Scientists of Sichuan Province (2010JQ0015).
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